1. Field of the Invention
The present invention is directed to a vacuum pumping apparatus.
2. Description of the Related Art
As a conventional vacuum pumping apparatus, a roots-type vacuum pumping apparatus is shown in FIG. 4. This conventional roots-type vacuum pumping apparatus includes a pair of intermeshed rotors 21a and 21b which rotate with a fixed phase difference maintained therebetween. When the rotors rotate, a gas is sucked into an inlet port (not shown) and is discharged from an outlet port (not shown) to create a vacuum. The rotors 21a and 21b are fixedly mounted on a driving shaft 29 and a driven shaft 30, respectively. The output shaft of a driving motor 24 is connected to a synchronizing gear 22a which is in meshing engagement with another synchronizing gear 22b. The synchronizing gear 22b is also in meshing engagement with a third synchronizing gear 22c. The synchronizing gear 22b is mounted to one end portion of the driving shaft 29, while the synchronizing gear 22c is mounted to one end portion of the driven shaft 30. Thus the driving force of the motor 24 is transmitted to the rotor 21a by way of the synchronizing gears 22a, the synchronizing gear 22b meshed therewith and the driving shaft 29 coupled thereto, thereby rotating the rotor 21a. Concurrently, the driving force transmitted to the synchronizing gear 22b is also fed to the rotor 21b by way of the synchronizing gear 22c meshed with the synchronizing gear 22b and the driven shaft 30, thereby rotating the rotor 21b. Due to the fact that the synchronizing gear 22b and the synchronizing gear 22c are meshed with each other, the rotor 21a is brought into synchronization with the rotor 21b, thereby establishing concurrent rotations of the rotors 21a and 22b with a fixed phase difference kept therebetween.
A lower portion of the synchronizing gear 22c is in a lubricating oil bath 25, and the lubricating oil 25 adhered to the synchronizing gear 22c is applied to the synchronizing gears 22b and 22a while the synchronizing gears 22a, 22b and 22c are in concurrent rotation, which ensures lubricating and cooling of the synchronizing gears 22a, 22b and 22c. In addition, for preventing the lubrication oil 25 from entering the pump housing 26, an oil seal member 28 is provided between the rotor 21a and the synchronizing gear 22b, and between the rotor 21b and the synchronizing gear 22c. 
A mechanical seal mechanism 23 is also placed between the motor 24 and gear chamber 27, and the driving force transmitting path passes through the seal mechanism 23. While the synchronizing gears 22a, 22b and 22c are in rotation, the gear chamber 27 accommodating the synchronizing gears 22a, 22b and 22c is in fluid communication with the interior of the housing 26 for the rotors 21a and 22b. As a result, both the housing 26 and the gear chamber 27 are at the low vacuum pressure. On the other hand, the motor 24 and its related portions are at atmospheric pressure. Thus, the mechanical seal mechanism 23 must prevent the atmospheric pressure from leaking into the gear chamber 27 and the housing 26.
In detail, the mechanical seal mechanism 23 includes a rubber member through which the output shaft of the motor 24 passes and an oil film extending between the rubber member and the output shaft of the motor 24. This means that the oil establishes a boundary lubrication condition between the rubber member and the output shaft of the motor 24, and the boundary lubrication condition assures the foregoing sealing function.
However, the mechanical seal mechanism 23 is relatively high in production cost. In addition, a small amount of gas leakage is inevitable in the mechanical seal mechanism 23, which results in air or atmospheric pressure leaking into the chamber 27 and the housing 26 in which the rotors 21a and 21b are accommodated, thereby lowering the vacuum producing ability of the vacuum pumping apparatus.
Japanese Patent Laid-open Publication No. Hei. 4 (1992)-31690 also discloses a vacuum pumping apparatus which is s similar to the above-described apparatus in concept, but is different therefrom in the number of synchronizing gears.
It is therefore a principal object of the present invention to provide a vacuum pumping apparatus without the foregoing drawbacks.
In order to attain the above and other objects, a vacuum pumping apparatus such as a pulse tube refrigerator includes a pump part including a pump housing, a pair of intermeshed rotors inside the pump housing, a pair of synchronizing gears for maintaining a phase difference between the rotors at a fixed value, the synchronizing gears meshed with each other and lubricated by oil, and an oil seal member preventing entry of the oil into the pump housing; and a motor part which is in the form of a sealed structure and is connected to the pump part in fluid-tight manner.